Electrostatic concentration of minerals



United States.

atent fifrce.

This invention relates to the electrostatic concentration of minerals,Moreparticularly, the invention,relatesto the restoration of .chargesusceptibility to frac: tions of rninerals which have beenrenderednon-responsive, to charging inter alia .by contact electrification.

Various procedures for thqelectrostaticconcentration of mineralsdifferentially charged. primarily by contact electrification aredescribed in Ljawver Patents 2,805 ,769' and. 2,847,147, LeBaron Patent2,738,875; and else-, wherein. the art. Such. mineral concentrationtech-, niques frequently yieldrniddling, concentrate or tail'fractionsunresponsive to further significant differential charg ing. Expedicntssuch as elevatedtemperatures andagitation effective initially to insurea charge differential appropriate for effective-electrostatic separationare ineffective when. applied. to such charge .noa: @p9 1siy fractions.Accordingly, conventional. procedures such as recycle of thenon-responsivefractions to, the dry head feed or the directsubjectionsuch fractions to additional stages of electrostatictreatment; are ineffective to achieve significant additionalbeneficiation.-

Various rocedures have been suggested for the restoration of. chargesusceptibility to non-responsive frac: tions resulting from priorpassage of a mineral mixture through-an- ;electrostatic;field. One suchprocedure described in Lawver Patent 2,723,029, entails exposing cleansurfaces predominantly by surface attrition of the particles of thenon-responsive fraction. An alternative procedure described in-La'wverPatent 2,805,770 contemplates mixture of non-responsive fractions withmaterials of a different work function. Yet another expedient;dis closedin Lawver Patent 2,832,469, contemplates washing ofsuch non-responsivefractions followed by drying of the washed fractions.

It is apparent-that the various; expedients suggested in the prior artfor the restoration of charge susceptibility to electrostaticallyseparated mineral fractions each entail serious economic disadvantagesinherent in the cost of attrition grinding, the provision, separationandrecycle of materials of different work function, washing and drying andthe like;

It is accordingly a primary object of this-invention to provide anovel-method effective to restore charge susceptibility tomineralfractions separated by electrostatic concentration.

It is a'more specific object-of the invention to provide a .novel methodeffective to restore charge susceptibility to electrostaticallyseparated fractions of relatively soft, attritionable minerals, such asphosphate rock.

It isanother object of the invention to provide an improvedelectrostatic mineral beneficiation procedure having part c larannlic iv o ph spha rook:

It is also a specific object of the invention to provide a noveltechnique forthe electrostatic concentration of minerals underconditions such. that .the mineral undergoes no significant chargemodification while in the electrostatic field.

It is an additional object of the invention to provide a method ofrestoring charge susceptibility tounreagentized charge non-responsivemineral mixtures.-

Itis more particularly an object ofthe-inventio'n to provide-an-improvedmethod for the electrostatic concentration of minerals in which theinitial head feed, at

one stage is wet, and a non-responsive fractionresult-' ing fromelectrostatic concentration is'recycledv to such wet head feed andtherebyagain-rendered susceptible to differential charging,

In accordance with this invention, a wet head feed comprising an orecomminuted to liberation-is dried, differentially charged and introducedinto an electrostatic field to produce'a concentratefraction and atleast one other fraction of impaired response to differential chargingrequisite to a further stage of electrostatic concentration, said otherfraction-is recycled to and commingled with said wet head feed to imparteffective charge susceptibility-to said fraction and said mixture of wethead feed and recycled fraction is differentially charged, andbeneficiated by passage through an electrostatic field.

It will be appreciated'that the invention-differs fundamentallyfromtechniques predicated upon chemical I reagent treatment to insureeffective differential'charging. Such a procedure is described; forexample, in Johnson Patent 2,197,865, which describes the recycle ofh anelectrostatic middling to a reagentizing operation inwhich thehead feedisconcur-rently treated.

Theinvention is generically applicable to theelectrostatic concentrationofall minerals, including. without limitation, all metallic andnonmetallic, silicate and nonsilicate-ores. Representative: mineralsinclude hematite, the various iron ores, copperores, mercury ores,-uranium-ores, and the other metallic ores, phosphate rock, calciticphosphate ores, lithium. ores such as spodumene, the Wariousfeldsparores, oil shale, and the like. The invention is particularly applicable:to the beneficiation oflph'osphate rock'ores such as Floridapebblephosphate.-

The invention is also generically applicable. in.-com-. bination'withall of the various techniques known to be useful. in electrostaticmineral concentration. A' preferred expedient entails heatingv the feedmineral. mixture to the electrostatic concentration step to a.temperature of t atleast. F., but below thedecomposition point of anycomponent of said mixture, inducing-the heated mineral mixture to acceptdifferential charges; while at a-temperature of at least-150? F;and;passing .the;differentially charged mixture through anelectrostaticfield While at a. temperature of at. least. 150 C. Suchprocesses are described in respect to: contact electrificationcharging-and so-called free fall electrostatic separation in LawverPatent 2,805,769, the disclosureofwhich is incorporated herein byreference.

It will be appreciatedthat the process-ofithe invention is embraciveof-the separation of particulate, mineral mixtures in electrostaticfields of all types and without regard to the nature or physicalstructure of theapparatus utilized. A preferred form of --apparatus. isof the free-fall type. which comprises spacedstationary electrodes ofopposite, signs-such asis disclosed; forexample, in FIGURE 3 of theLawverPatent;2,805,769.

The process of the invention, however, may also be practiced inroll-ty-pe. separators such as disclosed in Johnson. Patent 2,197,865,in which the. electrodes take the form of a ,rotating, relatively large,diameter roll by which the particles of ,thematerialsto be separated areconveyed into the field, and a relatively smaller electrode spaced fromthe roll. In respect to those minerals, which are mixtures ofnonconduotors, theaot-ual separation occurs inlsuch, roll separators insubstantially the same way as in free-fall apparatus utilizingstationary electrodes.

Such a result is a consequence of the fact that the nonconductingmineral particles-areeifectively charged substantially entirelybycontact electrification rather than by inductive conduction from theroll surface of a .roll separator as in the case of conductor particles.

The invention preferably is practiced .under conditions which effect nosignificant change in the charge on the Patented Mar'. 20, 1962';

particles undergoing separation during passage through the electrostaticfield, as would be occasioned, for example, by electron spray.

The invention generically contemplates electrostatic fields of anystrength requisite, as may readily be deterlmined by a person skilled inthe art, to eitect the desired separation between the charged particles.Generally the field gradient or strength may vary from about 1,000 voltsto about 15,000 volts per inch of distance between the electrodes. It ispreferred to utilize a total applied potential difference of 30,000 to250,000 volts maintained in a form of uniform potential substatniallyfree of alternating components. Filtered direct current low in alternating current ripple is appropriately utilized. Rectified radiofrequency power supplies are also appropriate.

In general, the particle size of the mixtures separated in accordancewit-h the invention should fall within the range of from about 4 toabout 200 mesh, preferably from about 8 to about 100 mesh. In general,comrninution of ores to a commercially practical degree of liberation issuitable. Removal of fines may be desirable but is not essential.

The conventional preparation of Florida pebble phosphate for the freefall electrostatic separation of apatite from silica is described interalia, in Lawver Patent 2,- 805,769. Such preparation entails washing,desliming and comminution to liberation. Accordingly, the invention in apreferred embodiment of the invention as applied to Florida pebblephosphate entails washing, desliming, and comminution to produce, as ahead feed, a wet substantially liberated phosphate pebble ore comprisinga mixture of phosphate and silica particles, drying and heating sandmixture to a temperature of at least about 150 F, differentiallycharging the phosphate and silica particles in said mixture while at atemperature of at least 150" F., passing the differentially chargedmixture through an electrostatic field to produce a concentrate fractionand at least one other fraction of impaired response to furtherdifferential charging, and recycling at least a portion of said chargenon responsive fraction of said wet head feed. Preferably the mixture ofparticulate phosphate and silica is charged and passed through theelectrostatic field at a temperature of from about 350 F. to about 550F.

The step of drying phosphate ores in preparation for difierentialcharging and electrostatic concentration preferably entails substantialagitation in a first stage to elfect high heat transfer and a minimum ofagitation in a final stage to improve the response of the dried ore tocharging particularly by contact electrification. Such drying techniquesare described in Lawver Patent 2,847,123.

The fraction of impaired charge susceptibility appropriately comprisesof from about 2% to about 80% and preferably from about 10% to about 50%by weight of the solids, in the mixture of said fraction with the wethead feed. The specific percent of recycle material in the wet admixturedepends, in general, on the operating conditions and the composition ofthe fresh feed.

The invention will be more fully understood from the following example.

EXAMPLE Florida pebble phosphate obtained as washer debris having a sizerange of from 1 millimeter to +150 mesh after having been through theusual deslirning operation in a classifier for flotation plantbeneficiation was dewatered and dried.

The dried solids, which had a BPL content of 27.4% by weight, weredelivered to a vibrating trough for discharge of the particles as freelyfalling bodies between the electrodes of an electrostatic separationunit having the electrodes spaced approximately 10 inches apart andmaintained at a potential gradient of approximately 9000 volts per inch.

A tail fraction from this first stage was passed through a second orso-called scavenger stage of separation, and a scavenger concentrate anda tail were separately recovered. The scavenger concentrate analyzed48.84% BPL. The result from passing the scavenger concentrate through athird stage of electrostatic separation are given below in Table 1.

From the previous listed data, it may be noted that 62.4 weight percentwas in the center pans (4 and 5) which indicates that this material wassubstantially nonresponsive to the electrostatic field.

In a series of tests, various amounts of the scavenger concentrate wererecycled and mixed with the wet solids from the classifier which has awater content of about 18%. The wet mixtures Were dried at 300 F. fortwo hours, charged and subjected to an electrostatic beneficiationoperation as above described. The amount of scavenger concentrate mixedwith the fresh feed was varied as indicated in Table II.

Table II Wt. Wt. percent percent Test No. Fresh Scavenger Pan N o. Wt.Percent Feed Coucenpercent BPL trate 2 st 5a 1 0 5 1312 44'. 81 6-8 27.6 66. 72

2 5 13. 4 50. 68 6-8 28. 6 68. 51 ii'g iii? 3 5 12. 6 50. 09 6-8 33. O69. 01 i3? 1% ii 5 5 l9. 3 56. 51 6-8 30. 4 69. 78 at at Analogusresults are afiected with the other ores contemplated by the inventionand with each of the various types of apparatus with which the inventionmay be practiced.

Having now fully described and illustrated the invention, what isdesired to be secured and claimed by Letters Patent is set forth in theappended claims.

1. A process for the electrostatic concentration of minerals withoutchemical reagentizing consisting essentially of providing a wet headfeed of an ore comminuted to liberation, drying said head feed,imparting a differential charge to the particulate value and ganguecomponents of said dried head feed, introducing said differentiallycharged particles into an electrostatic field to produce a concentratefraction and at least one other fraction of impaired response todifferential charging requisite to a further stage of electrostaticconcentration, forming a mixture of said other fraction with said wethead feed to impart efiective charge susceptibility thereto, drying, anddifferentially charging said mixture, and beneficiating '5 said driedand charged mixture by passage through an electrostatic field.

2. The process of claim 1 wherein said head feed and said mixture aredifferentially charged by contact electrification.

3. The process of claim 2 wherein said dried and difierentially chargedhead feed is passed as freely falling bodies through said electrostaticfield to produce said concentrate fraction and said other fraction.

4. The process of claim 3 wherein said mixture is beneficiated bypassage through said electrostatic field as freely falling bodies.

5. The process of claim 1 wherein said head feed is subjected to anelectron spray, and said concentrate fraction and said other fractionare produced by passage of said head feed through a roll-typeelectrostatic separator.

6. The process of claim 5 wherein said mixture is charged by an electronspray and said mixture is beneficiated by passage through a roll-typeseparator.

7. The process of claim 1 wherein said ore is a phosphate ore.

8. The process of claim 1 wherein said ore is Florida pebble phosphate.

9. The process of claim 1 wherein said ore is an iron ore.

10. The process of claim 1 wherein said ore is hematite.

11. The process of claim 1 wherein said ore is comminuted to liberationat a mesh size of from about eight to about one hundred.

12. A process for the electrostatic concentration of phosphate rock orewithout chemical reagentizing consisting essentially of washing,desliming and comminuting said ore to provide as a head feed, a wetmixture of phosphate and silica particles, drying and heating saidmixture to a temperature of at least 0 F., differentially charging thephosphate and silica particles in said mixture while at a temperature ofat least 150 F., passing the differentially charged mixture into anelectrostatic field while at a temperature of at least 150 F. to producea concentrate fraction and at least one other fraction of impairedresponse to further difierential charging, and recycling said otherfraction to said wet head feed.

13. The process of claim 12 wherein phosphate and silica particles insaid mixture are diflerentially charged by contact electrification.

14. The process of claim 12 wherein said phosphate and silica particlesin said mixture are differentially charged and passed into saidelectrostatic field at a temperature of from about 350 F. to about 550F.

15. The process of claim 12 wherein said other fraction is recycled inan amount requisite to constitute from about two percent to about eightypercent of the total weight of the solids in the mixture of saidfraction with said head feed.

16. The process of claim 12 wherein said other fraction is recycled inan amount requisite to constitute from about ten percent to about fiftypercent of the total weight of the solids in the mixture of saidfraction with said head feed.

17. The process of claim 12 wherein said diflferentially charged mixtureof phosphate and silica particles is passed through said electrostaticfield as freely falling bodies.

18. The process of claim 12 wherein said differentially charged mixtureof phosphate and silica particles is conveyed into said electrostaticfield by a rotating roll of a roll-type electrostatic separator.

19. The process of claim 12 wherein the field gradient of saidelectrostatic field is from about 1000 volts to about 15,000 volts perinch of distance between the electrodes.

20. The process of claim 12 wherein the differential charge on thephosphate and silica particles is not significantly changed duringpassage through said electrostatic field.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR THE ELECTROSTATIC CONCENTRATION OF MINERAL WITHOUTCHEMICAL REAGENTIZING CONSISTING ESSENTIALLY OF PROVIDING A WET HEADFEED OF AN ORE COMMINUTED TO LIBERATION, DRYING SAID HEAD FEED,IMPARTING A DIFFERENTIAL CHARGE TO THE PARTICULATE, VALUE AND GANGUECOMPONENTS OF SAID DRIED HEAD FEED, INTRODUCING SAID DIFFERENTAILLYCHARGED PARTICLES INTO AN ELECTROSTATIC FIELD TO PRODUCE A CONCENTRATEFRACTION AND AT LEAST ONE OTHER FRACTION OF IMPAIRED RESPONSE TODIFFERENTIAL CHARGING REQUISITE TO A FURTHER STAGE OF ELECTROSTATICCONCENTRATION, FORMING A MIXTURE OF SAID OTHER FRACTION WITH SAID WETHEAD FEED TO IMPART EFFECTIVE CHARGE SUSCEPTIBILITY THERETO, DRYING, ANDDIFFERENTIALLY CHARGING SAID MIXTURE, AND BENEFICATING SAID DRIED ANDCHARGED MIXTURE BY PASSAGE THROUGH AN ELECTROSTATIC FIELD.